1. Technical Field of the Invention
This invention relates to electroacoustic transducers for accurate reproduction of sound. Preferred embodiments of the invention use a layered assembly of thin films, baffles and atmospheric air, as an oscillating medium.
2. Description of Prior Art
Transducers for accurate or high-fidelity reproduction of sound available on the market today, when defined by the kind of oscillating medium they use, are of rigid-diaphragm type or stretched thin-film type:
1. As a rule, rigid-diaphragm transducers are employed in the construction of transducers with enclosures. They mostly consist of a cone-shaped radiating area with an electromagnetic driven area (voice coil) positioned at the apex of the cone. The front of the radiating area is fully exposed, whereas at the back of such area there is a magnet assembly and supporting hardware. PA1 2. Stretched thin-film transducers do not employ any enclosure. They are mostly rectangular shaped with either an electrostatic or electromagnetic driving (motor) structure distributed over the entire front or back (or front and back) of the driven area. PA1 (a) Concentration of exerted forces only on a symmetrically positioned, with respect to boundary, central and small section of an oscillating medium, such as in rigid diaphragm transducers; PA1 (b) Distribution of exerted forces throughout the entire area of an oscillating medium, including the central section, such as in thin-film transducers. PA1 (a) such area being, to a large extent, symmetrically crossed by nodal lines, PA1 (b) such nodal lines belonging to a large number of mode patterns, PA1 (c) such mode patterns having an even number of antinodes, PA1 stimulus-coupling means for asymmetrically coupling said stimulus with said oscillating medium, so as to obtain an initially high effective density of intrinsic normal modes of said oscillating medium concurrently participating in the reproduction of said stimulus; and PA1 stimulus-driving means for driving said stimulus-coupling means, so as to exert linear and phase-coherent forces combined with improved exchange of energy between the oscillating medium and said ambient medium; PA1 whereby the spectrum of the reproduced waveform of said effective output of said transducer constitutes a satisfactorily faithful reproduction of the spectrum of the waveform of said stimulus at source. PA1 an antinode area belonging to a predetermined mode pattern of each said at least two oscillating components of said oscillating medium, asymmetrically positioned therein; PA1 at least one inner baffle, parallel with said at least two oscillating components while suspended at predetermined inner-air-gap distance therebetween, said inner baffle having an asymmetrically positioned opening (open area) therein aligned with said antinode area, for mutually and asymmetrically coupling each of said at least two oscillating components at said antinode area, while mutually decoupling each said at least two oscillating components in the remaining area; PA1 so as to obtain an initially high number of superposed standing waves concurrently excited in said oscillating medium resulting from said applied thereupon stimulus. PA1 two parallel outer baffles, with said oscillating medium suspended therebetween at predetermined outer-air-gap distance, each of said outer baffles having an asymmetrically positioned opening (open area) therein aligned with said antinode area, for asymmetrically coupling said oscillating medium with said ambient medium by way of said asymmetrically positioned antinode area and through said opening, while decoupling said oscillating medium from said ambient medium in the remaining area, so as to limit or minimize occurrences of resonance-antiresonance irregularities in effective output. PA1 perturbations in geometry of said at least two oscillating components, altering said boundary conditions of said oscillating medium, defining oscillating regions therein having distinct normal modes of oscillation and associated characteristic frequencies interlaced in the frequency domain for further increasing said effective density of intrinsic normal modes of said oscillating medium so as to limit or minimize detection of resonance-antiresonance irregularities in effective output. PA1 each of said thin films stretched with predetermined tension on a rectangular inner-baffle spacer of predetermined thickness, bordering each side of said inner baffle, also defining boundary geometry of each of said thin films; PA1 said predetermined tension for defining interlacing positions in the frequency domain of characteristic frequencies associated with the normal modes of oscillation of each of said thin films; PA1 each of said thin films being suspended parallel with, and at inner-air-gap distance, from said inner baffle by way of said inner-baffle spacer, while forming with said inner-baffle spacer and said inner baffle, in combination, a hermetically sealed cavity with air entrapped therein; PA1 said thin films being coupled at said antinode area, through said inner-baffle opening, by air entrapped in said cavity, for maximizing interaction of modes in said antinode driven area; PA1 said inner baffle decoupling said thin films in the remaining nondriven area, for limiting interaction of modes thereof; PA1 air oscillating in said cavity substantially matching the impedance of said composite oscillating medium with air of said ambient medium, for optimizing acoustic energy exchange between said oscillating medium and said ambient medium; PA1 whereby said oscillating medium constitutes a layered module of two oscillating thin films forming, in combination with said inner baffle and said inner spacer an oscillating air-sealed cavity coupling asymmetrically said two oscillating thin films. PA1 controlling means for controlling the intrinsic normal modes of oscillation of said oscillating medium, so as to limit effective output irregularities caused by resonance frequencies, associated with said normal modes, participating in the spectrum of a waveform reproduced from said applied stimulus; PA1 matching means for matching impedance of said oscillating medium with said ambient medium so as to optimize exchange of energy between said oscillating medium and said ambient medium; and PA1 driving means for linear and in-phase driving of said oscillating medium by said stimulus, so as to limit effective output irregularities caused by nonlinear as well as out-of-phase displacements of said oscillating medium; whereby the spectrum of the reproduced waveform of said effective output of said transducer constitutes an improved approximation of the spectrum of the waveform of said stimulus at source. PA1 stimulus-interfacing means for asymmetrically and partially coupling said oscillating medium with said stimulus so as to obtain an initially high density of normal modes concurrently participating in reproducing said stimulus; PA1 environment-isolating means for asymmetrically and partially decoupling said oscillating medium from said ambient medium, so as to limit occurrences of resonance-antiresonance irregularities in effective output. PA1 mode-density means for substantially increasing said effective density of normal modes of said oscillating medium concurrently participating in reproducing said stimulus, so as to limit detection of resonance-antiresonance irregularities in effective output; PA1 an asymmetrically positioned antinode belonging to a predetermined mode pattern of said oscillating medium, for asymmetrically and partially coupling said oscillating medium to said stimulus, so as to obtain a substantially high effective density of normal modes concurrently participating in reproducing said stimulus. PA1 A composite oscillating medium having at least two oscillating members coupled at said antinode while decoupled in the remaining area of said oscillating members, each of said oscillating members having distinct normal modes of oscillation and having associated characteristic frequencies interlaced in the frequency domain, for further increasing the effective density of modes. PA1 an asymmetrically positioned antinode belonging to a predetermined mode pattern of the oscillating medium, for asymmetrically and partially coupling said oscillating medium to said stimulus, so as to obtain a substantially high effective density of normal modes concurrently participating in reproducing said stimulus; PA1 at least one outer baffle facing the oscillating medium and separated from the oscillating medium by a predetermined air-gap distance, said baffle having an asymmetrically positioned opening therein aligned with said antinode, so as to allow asymmetrical and partial coupling of the oscillating medium with said ambient medium through said opening. PA1 tonal quality irregularities caused by the normal modes of a symmetrically driven oscillating medium; PA1 audibility of resonance-antiresonance irregularities caused by the normal modes of the oscillating medium; PA1 occurrences of resonance-antiresonance irregularities caused by lower frequency modes of the oscillating medium; PA1 coloration caused by resonances of structural components; and/or PA1 irregularities caused by resonances of complex circuitry. PA1 a magnet array structure interfering with radiation of emitted waves; PA1 a magnet array structure with inherent air flow impedance interfering with the free motion of the coupled with air driven/radiating area; PA1 an oscillating medium driven by strips of adhered thereon conductors oscillating out of phase with nondriven sections thereof; PA1 an oscillating medium flapping against the magnet array structure at high amplitude low frequency displacements. PA1 directionality of radiated sound; PA1 acoustical properties of the listening room; PA1 back-to-front-wave-leakage-phase cancellations; PA1 back-to-front-wave-path destructive interference. PA1 nonlinearity of forces exerted on current-carrying conductors; PA1 weak mechanical coupling of conductors with driven/radiating area; PA1 sparse distribution of conductors over driven/radiating area; PA1 losses in impedance matching transformers; PA1 losses in complex circuitry of filters and crossover networks; PA1 sparse distribution of the magnetic field and of the resulting forces exerted throughout the driven/radiating area; PA1 magnetic flux losses. PA1 the asymmetric coupling of the oscillating medium with ambient medium, PA1 the asymmetric coupling of oscillating thin films with each other, PA1 means for increasing normal modes density by interlacing resonance frequencies, PA1 driving an oscillating thin film at asymmetrically positioned antinode, PA1 the inner baffle as a non-oscillating component of the oscillating medium, PA1 the outer baffles as means for filtering out resonance irregularities, PA1 the inner and outer baffles contributing to definition of boundary conditions, PA1 the grid of conductors, PA1 the grid-shaped magnetic field, PA1 the hermetically sealed cavity with magnet array `immersed` therein, PA1 linearity of exerted forces due to cavity-`immersed` magnetic field, PA1 phase-coherent forces exerted from constant pressure inside the cavity, PA1 impedance matching of oscillating with ambient medium by way of air in cavity, PA1 the absence of any driving components in front and back of driven area. PA1 providing a transducer having an oscillating medium with a driven/radiating area therein defined by an asymmetrically positioned antinode belonging to a predetermined mode pattern, for limiting interference in the development of normal modes from disturbances that act in conflict with out-of-phase displacements of adjacent antinodes; the resulting high number of normal modes that participate in the process of reproducing the spectrum of a radiated waveform, leads to an effective output rich in overtones; PA1 providing a transducer having an oscillating medium with modified effective dimensions from perturbations in boundary shape that cause multiple path, multiple length reflections of a propagated disturbance, for producing additional superposed standing waves; the resulting increased effective mode density limits perception of resonance-antiresonance irregularities and extends the effective frequency range of the transducer. PA1 providing a transducer with an oscillating medium and a driven/radiating area therein defined by an asymmetrically positioned antinode belonging to a predetermined mode pattern, such oscillating medium suspended in the gap between two parallel baffles with asymmetrically positioned openings therein, such driven/radiating area aligned with, and interacting through, such openings with the ambient medium; the consequent masking of the oscillating medium nondriven area limits occurrences of resonance-antiresonance irregularities caused by the lower frequency normal modes, including the pronounced irregularity at the fundamental resonance frequency; PA1 providing a transducer having component baffles with mutually destructive interference of inherent normal modes, for reducing coloration caused by resonances of such baffles; PA1 providing a transducer having an oscillating medium driven by a simple, resistive circuit, with terminals directly coupled to a signal amplifying device, for eliminating inherent resonances of complex circuitry. PA1 providing a transducer having the magnet array structure positioned within the driven/radiating area of the oscillating medium, for eliminating interference with propagation of emitted waves; PA1 providing a transducer with grid-like magnet-array gaps giving rise to low air flow impedance, for reduced interference with the free motion of the coupled with air driven/radiating area; PA1 providing a transducer having an oscillating medium with a driven/radiating area oscillating in-phase for every point thereof; PA1 providing a transducer having an oscillating medium with an asymmetrically positioned driven/radiating area therein combined with an asymmetrically positioned boundary-perturbation and with reduced compressibility of air in multiple air-gaps between the layers of the oscillating components and the layers of the supporting baffles, for increased protection from relatively small driven/radiating area, flapping against the magnet array structure. PA1 providing a transducer with a baffle area having adjustable orientation for controlling directionality of radiated sound; PA1 providing a transducer having an oscillating medium with air entrapped therein, suspended in the air gap between two parallel baffles, with each of such baffles having an asymmetrically positioned and limited size opening therein, for reduced dependence on the acoustical properties of the listening room, that is, for restricting irregularities from the interaction of the air volume normal modes with the oscillating medium motion; such oscillating medium interacts with the ambient air through each of such openings as well as by way of such entrapped air and by way of air in such air gap, for improved acoustic impedance matching with ambient air; PA1 providing a transducer having a driven/radiating area aligned with an opening in a baffle, such baffle being relatively large with respect to such opening, for restricting irregularities from the back-to-front-wave-leakage-phase cancellations; PA1 providing a transducer having a driven/radiating area aligned with an opening in a baffle, such opening asymmetrically positioned in such baffle, for restricting irregularities from the back-to-front-wave-path destructive interference. PA1 providing a transducer with a magnet array structure having a practically constant magnetic field, for exerting linear push-pull forces on current-carrying conductors within limits of high amplitude excursions of driven/radiating area; PA1 providing a transducer having a driven/radiating area with conductors adhered thereon forming a grid-shaped assembly, for strong mechanical coupling of such conductors with such driven/radiating area; PA1 providing a transducer having a driven/radiating area with conductors adhered thereon forming a grid-shaped assembly, for a more uniform distribution of electric current in such conductors over such driven/radiating area; PA1 providing a transducer having a driven/radiating area with conductors adhered thereon forming a grid-shaped assembly resulting in higher impedance from a longer conductor path, for matching from the outset the impedance of a signal amplifying device; PA1 providing a transducer having an oscillating medium driven by a simple, resistive circuit, with terminals directly coupled to a signal amplifying device, for preventing inherent distortions of complex circuitry; PA1 providing a transducer with a grid-shaped magnet array forming a grid-shaped magnetic field that is aligned with the grid-shaped current-carrying conductors, for a more uniform distribution throughout the driven/radiating area of the magnetic field and of the resulting exerted forces; PA1 providing a transducer with a grid-shaped magnet array structure using no pole piece, for a minimum in magnetic flux losses; PA1 said ambient medium is atmospheric air; PA1 said effective range frequencies are within normal human hearing limits; PA1 said oscillating medium is a rectangular thin film coupled to and driven by said stimulus at said asymmetrically positioned antinode. PA1 a layered assembly of two rectangular parallel oscillating thin films with a substantially massive, rigid, rectangular inner baffle therebetween, having outside dimensions corresponding to the dimensions of said thin films; PA1 said inner baffle having a rectangular inner-baffle opening asymmetrically positioned therein and aligned with said asymmetrically positioned antinode; each of said thin films being stretched with predetermined tension on a rectangular inner-baffle spacer of predetermined thickness, bordering each side of said inner baffle and defining boundary geometry of each of said thin films; PA1 said predetermined tension defining interlacing positions in the frequency domain of characteristic frequencies associated with the normal modes of oscillation of each of said thin films; PA1 each of said thin films being suspended parallel with, and at air-gap spacing, from said inner baffle by said inner-baffle spacer, while forming in combination with said inner-baffle spacer and said inner baffle, a hermetically sealed cavity with air entrapped therein; PA1 said thin films being coupled at said antinode, through said inner-baffle opening, by air entrapped in said cavity, for maximizing interaction of modes in said antinode driven area; PA1 said inner baffle decoupling said thin films in the remaining nondriven area, for limiting interaction of modes thereof; PA1 air oscillating in said cavity substantially matching the impedance between said oscillating medium and air of said ambient medium, for optimizing acoustic energy exchange between the two media; PA1 whereby said oscillating medium constitutes a layered module of two oscillating thin films with an inner baffle therebetween forming an oscillating air-sealed cavity coupling asymmetrically and partially said two oscillating thin films. PA1 clamps (desirably right-angle triangles) introduced as predetermined perturbations in geometry of each of said thin films, which alter said boundary conditions of said oscillating medium and define distinct oscillating regions in each of said thin films with additional said characteristic frequencies interlaced in the frequency domain; and PA1 rectangular clamps (desirably narrow) introduced as variable perturbations in geometry of said oscillating thin films, which alter the area size of each of said oscillating regions and fine-tune, by shifting, the interlaced positions of said characteristic frequencies in the frequency domain for each of said oscillating regions; PA1 whereby said boundary conditions of said oscillating medium are component-determined by said inner baffle in combination with each said inner-baffle spacer and said perturbations in geometry of said oscillating medium. PA1 a pair of parallel outer baffles of shape and size substantially identical with said inner baffle, each of said outer baffles having an asymmetrically positioned outer-baffle opening therein and a rectangular outer-baffle spacer of predetermined thickness bordering the inner side of each of said outer baffles; PA1 a sandwich configuration of said two outer baffles with said oscillating medium suspended at air-gap spacing therebetween by said outer-baffle spacers, and having said asymmetrically positioned outer-baffle opening therein aligned with said antinode for asymmetrically and partially coupling said oscillating medium with said ambient medium; PA1 whereby air oscillating in said air-gap spacing affects advantageously the impedance matching between said oscillating medium and air of said ambient medium, for improved acoustic energy exchange between the two media. PA1 a hinged baffle extending the area of said outer baffles while supporting the transducer in upright position; PA1 whereby said boundary conditions of said oscillating medium are physically determined by the combination of said outer baffles, said inner baffle, each said outer-baffle spacer, each said inner-baffle spacer, air in each air-gap spacing, said perturbations in geometry of said oscillating medium and said hinged baffle extension. PA1 an assembly of current-carrying grid of conductors secured to each said antinode; PA1 a frame-like magnet array, flush-mounted in each said outer-baffle opening; PA1 a panel-like magnet array flush-mounted in said inner-baffle opening; PA1 said frame-like magnet array and said panel-like magnet array being so disposed as to expose concurrently and in combination said grid of conductors of each said antinode to a substantially uniform magnetic field within the space defined by extreme amplitude displacements of each said antinode, so that forces exerted on said current-carrying conductors are substantially linear; PA1 said air-sealed cavity in said oscillating medium having constant air pressure from said two thin films moving in tandem so that forces per area unit of oscillating medium are uniform and in-phase; air in said cavity having substantially constant pressure for uniform and in-phase oscillating forces per unit area of said antinode of each of said thin films; PA1 whereby said driving means are external and internal elements of said oscillating medium.
I will first examine deficiencies common to all transducers. Then I will cover deficiencies or problems typical of each of the two kinds of transducers with emphasis on stretched thin-film transducers. Examined deficiencies are mostly confined to physical properties and phenomena causing audible irregularities in frequency response and tonal quality (timbre) of reproduced effective output.
A. Deficiencies Common to All Transducers, Affecting Perceived Tonal Quality
A first deficiency common to all transducers is the erratic acoustic power response as a function of frequency. Such erratic response is the result of irregularities in radiated output caused by characteristic mode patterns, known as normal modes of oscillation, resulting from standing waves at resonance frequencies of an oscillating medium. Such mode patterns are determined by oscillating sections of maximum displacement known as antinodes that are delimited by lines of zero displacement known as nodes. The areas of any adjacent antinodes of an oscillating medium have the tendency to be equal in size. Such tendency is a function of the geometry of the oscillating medium boundary in the sense that the closer to a symmetric geometry such boundary is, the more pronounced such tendency is. Each antinode moves out of step or with 180 degrees phase difference with any adjacent antinode. Moreover, the acoustic power radiated by any section of an oscillating medium is a function of the average displacement amplitude of such section. Thus, for each pair of adjacent antinodes, separated by a nodal line, the resulting minimum in the average displacement causes a drop in the effective output. By extrapolation, for each resonance (characteristic) frequency of an oscillating medium producing a mode pattern with an even number of antinodes, there is a minimum average displacement giving an audible dip in effective output. Such dip in effective output is defined as an antiresonance minimum. For a mode pattern with an odd number of antinodes, there is an audible peak in the effective output determined by a maximum from a remaining single (not paired up) antinode. Such peak in effective output caused by a mode pattern with a maximum average displacement is defined as a resonance maximum.
The irregularities in effective output of resonance-antiresonance minima and maxima become particularly audible in the low end of the acoustic spectrum where mode patterns occur at wider spaced resonance frequencies of the oscillating medium. The sparser such resonance frequencies are, the fewer mode patterns occur per unit frequency range, and the more pronounced the effect of such irregularities is, as perceived by the hearing mechanism. One more reason for strong and audible resonance-antiresonance irregularities from mode patterns is the stronger coupling with air of the intrinsically larger antinode areas occurring at low resonance frequencies. Consequently, not only the acoustic power response is erratic but also tonal quality (timbre) of radiated sound deteriorates because dips and peaks, unrelated to the spectrum at source, occur in the reproduced complex sound such as music or speech. Such resonance-antiresonance irregularities are also particularly pronounced under transient conditions at all frequencies.
A second deficiency common to all transducers relates to the tonal quality or timbre of reproduced sound as affected by, correspondingly, size and position of the driven area with respect to size and boundary of the total oscillating area. To a large extent, tonal quality is determined by the structure of the spectrum as derived from a reproduced complex waveform. In turn, a reproduced waveform is affected by the coexistence of superposed standing waves of an oscillating medium at any instant. Therefore such deficiency is reduced to relating tonal quality directly to the content in modes of oscillation being simultaneously excited in the oscillating medium.
A disturbance at a point of an oscillating medium will excite simultaneously a number of modes in proportion to the amplitude associated with each mode pattern at that particular point. One extreme possibility would be for a disturbance, applied at a point of maximum displacement--the centre of an antinode, giving rise to a pronounced associated mode. Another extreme possibility would be for a disturbance, applied at a point of minimum displacement--a nodal line, not being able to excite at all the associated mode. In practice, a disturbance spans at least two adjacent antinodes and interferes with the formation of associated mode patterns because the applied forces act in conflict with the adjacent out of step displacements.
Established and prevalent driving configurations in transducers involve:
Either way, at lower frequencies, a centrally driven section of an oscillating medium is crossed by nodal lines delimiting relatively large adjacent antinodes. The out-of-step displacements of adjacent antinodes conflict with forces applied by a disturbance and restrict the full development of associated mode patterns. Hence, a waveform reproduced by a centrally driven oscillating medium will have spectrum poor in low frequency overtones, causing tonal quality irregularities in effective output. At higher frequencies, progressively denser nodal lines delimit progressively smaller antinodes, and the effect of disturbances conflicting with out-of-phase antinode displacements becomes only statistically significant throughout the entire oscillating medium.
A third deficiency common to all transducers is related to back-to-front-wave-leakage phase cancellations whereby a back compression or a back rarefaction leaks around the edge of the transducer and catches up, respectively, with a front rarefaction or a front compression. The back-to-front-wave-leakage phase cancellations are an additional cause of irregularities in effective output, phenomenon particularly pronounced at low frequencies.
A fourth deficiency is related to all transducers using more than one transducer units in order to extend the frequency range of reproduced sound. Filters (crossover networks) employed in distributing frequency bands to dedicated transducer units, are circuits with their own resonances and losses that affect the original waveform, therefore affecting the perceived tonal quality reaching the listener's ears. A complex problem to solve is the integration of two adjacent frequency bands of two transducer units at frequencies delimiting such bands. Because both such units radiate sound at such delimiting frequencies, the transition from one band to the other must be smooth, or output discontinuity will occur. Crossover networks employed to handle such transition remain a major source of complications in preventing this irregularity.
A fifth deficiency common to all transducers is nonlinear distortion. Nonlinear distortion is due to nonlinear mechanical properties such as elasticity of oscillating media, nonlinear electrical properties such as impedance of crossover networks and voice coils, nonlinear electromagnetic or electrostatic fields producing nonlinear forces exerted on oscillating media. Improvements in material technology (thin film, diaphragm, diaphragm suspension) have reduced nonlinear distortion from mechanical properties to levels difficult to detect in perceived tonal quality. Audible forms of nonlinear distortion are mostly due to nonlinearity of crossover circuits, in general, and, in particular, of voice coils in magnetic fields of thin-film transducers with the magnet structure mounted on one side of such thin film.
A sixth deficiency common to all transducers is directionality. At high frequencies, the acoustic energy is mostly propagated in the direction perpendicular to the plane of the oscillating medium. The larger the ratio of oscillating area dimensions to wavelength of propagated sound, the more pronounced such directionality is.
Note: For all types of transducers, limited attention has been paid to the importance of direct radiation as related to early reflections of emitted sound or to the balance between a listener's ability to localise a sound source and the spatial sensation of the source environment; too much dispersion, and definition is lost; too much directionality, and spaciousness is lost.
B. Deficiencies Typical of Each Kind of Transducers Affecting Perceived Tonal Quality
1. Rigid-diaphragm Transducers
Rigid-diaphragm transducers are of a relatively high sensitivity, requiring small amounts of power to produce acceptable acoustic output. As standalone devices, they produce sound of poor tonal quality because of every possible phase cancellation. When mounted in enclosures, however, rigid-diaphragm transducers produce a pleasant (warm) sound but with an artificially rich tonal quality, an inevitable departure from natural, life-like sound reproduction.
A first deficiency typical of rigid-diaphragm transducers is instability of diaphragm. No matter how rigid the diaphragm is made, it is impossible to eliminate or even control its break up (flexing) at resonance frequencies. Especially at higher frequency normal modes of oscillation it is impossible to predict the behaviour of the diaphragm since it ceases following the motions of the voice-coil. Low-mass-high-stiffness materials have been about the best means in constructing the diaphragm. A major obstacle has been the unavoidable compromise between stiffness and low mass. For a given diaphragm mass, the stiffer the material, the less bending (flexing) occurs at resonance frequencies and the more faithfully will the diaphragm follow the motions of the driven area.
A second deficiency of rigid-diaphragm transducers is a frequency-dependent output because of high electric impedance of the coil (voice-coil) driving the diaphragm. This makes the current intensity highly dependent on frequencies present in the waveform, therefore affecting acoustic output and tonal quality. Voice-coil impedance remains a limiting factor in producing frequency-independent output. Compromises between low impedance, electromagnetic force and coil mass remain a major obstacle.
A third deficiency of rigid diaphragm transducers is the poor exchange of energy, between the diaphragm (high density) as the oscillating medium and air (low density) as the ambient wave propagation medium, caused by the impedance mismatch of the two wave-carrying media.
A fourth deficiency is typical of rigid-diaphragm transducers mounted in enclosures. A sealed enclosure eliminates back-to-front-wave-leakage phase cancellations, acting as an approximation of an infinite baffle. An open enclosure minimizes back-to-front-wave-leakage phase cancellations while acting as a low frequency resonator (Helmoltz) and as an acoustic transformer between the diaphragm and the ambient air through the enclosed volume of air. An open enclosure solves partially the low frequency phase cancellation problem and the impedance matching problem but it creates a new one: coloration. The reproduced effective output is altered (coloured) by added resonances occurring in the enclosed volume of air that have no relation to the waveform at source, giving rise to an artificially rich tonal quality. Also, the panels of any enclosure become another source of spurious resonances. Damping air resonances has had limited success in harnessing coloration. It has been proven that although damping reduces ringing by lowering and broadening the resonance peaks (lowering the quality factor Q), it does not reduce audibility of resonances (R. Bucklein--1962, The Audibility of Frequency Response Irregularities, J. Audio Eng. Soc., vol.29, pp.126-131, March 1981). Rigid and damped panels with internal bracing help in building enclosures with excellent properties, yet a barrel-like-originating sound persists in all transducers using enclosures.
2. Stretched Thin-film Transducers
Although able to reproduce low-coloration and low-distortion output, stretched thin-film transducers tend to sound dry, unbalanced, as if lacking the low frequency end of the spectrum. Such transducers employ a relatively large driven (and radiating) area that also requires large and, often, expensive magnetic or electrostatic motor structures. The relatively large driven/radiating area is the source of most of their deficiencies.
A first deficiency typical of thin-film transducers is an erratic response in coustic power, with pronounced audible effects, caused by resonance-antiresonance irregularities. Such irregularities are more pronounced because of a typically large and symmetrical area of the oscillating thin film. The generally accepted reason for making such area large is to compensate for small amplitude displacements. For given amplitude, a larger area displaces larger volumes of air, and, in principle, this is true for any non-resonance frequencies. However, at the occurrence of each mode pattern, resonance-antiresonance irregularities cause excessive variations in air volume displacements, giving rise to pronounced variations in effective output, especially audible at low frequencies. This is because, for a predetermined thin-film area, the lower the resonance (characteristic) frequency, the larger the areas of antinodes and the stronger the coupling with air.
One method to alleviate the problem of resonance-antiresonance irregularities, used by Magnepan Co. and disclosed in U.S. Pat. No. 4,319,096 to J. M. Winey (1982), the disclosure of which is incorporated herein by reference, is by clamping the thin film at points causing excessive amplitude fluctuations. The effectiveness of the method is limited to specific resonance frequencies.
A proposed method of reducing the effect of normal modes is by applying a lightweight damping material on the thin film, as disclosed in H. Suyama (1981). Besides the problem of added mass, bending and flexing at resonance frequencies still occurs, therefore, audible improvements in tonal quality are minor.
A second deficiency typical of stretched thin-film type of transducers is that they behave as large planar dipoles emitting acoustic energy in front and back with unavoidable back-to-front-wave-leakage phase cancellations at low frequencies, whereby a compression generated at the back catches up with a rarefaction generated at the front of the dipole. This deficiency reduces acoustic power, and is particularly noticeable at the low end of the spectrum, resulting in unbalanced sound, lacking low frequency content. There is virtually no baffle to prevent back-to-front-wave-leakage phase cancellations because this would make their total size prohibitive to normal domestic or professional use.
Increasing the oscillating area offers some marginal benefits to perceived effective output because a larger oscillating area means also a larger, bordering the edge, peripheral area that acts as a virtual baffle for the central area. Virtual baffle in the sense that the intrinsic stiffness of the peripheral area dictates low amplitude oscillations relative to the amplitude oscillations of the central area. Nevertheless, antiresonance minima reduce the low frequency content of the effective output to such an extent that the peripheral area ceases to play any significant role as a virtual baffle in preventing back-to-front-wave-leakage phase cancellations. On the other hand, such marginal benefits are outweighed by the inconvenience of an increased size and associated increased material and manufacturing costs.
A third deficiency of stretched thin-film type of transducers is their pronounced directionality. Because of a large oscillating area, the acoustic energy is mostly propagated in the direction perpendicular to the plane of the oscillating medium. The larger the oscillating area and the higher the frequency, the more pronounced such directionality is. Effective (and expensive) solutions for the directionality problem exist on the market. One product that solves the problem for horizontal and vertical dispersion of sound is the electrostatic ESL-63 by QUAD Electroacoustics. Using a single stretched thin film and circuitry feeding the signal with time delays, it simulates a hemispherical wavefront seeming to originate from a point source behind the oscillating medium. Other models (Acoustat, Martin-Logan) simulate a semicylindrical wavefront originating from a line source with acceptable horizontal but poor vertical dispersion of sound.
A fourth deficiency typical of thin-film transducers is, again, caused by the large driven/radiating area as related to the way standing waves and resulting mode patterns are excited at resonance frequencies of the air in the listening room. The larger the driven/radiating area, the higher the probability of such area spanning room air volumes (typically mode patterns with antinodal parallelepipeds and nodal planes) that oscillate out of step at resonance frequencies. The equivalent statement would be that the larger the driven/radiating area, the higher the probability of such area being crossed by a nodal plane separating two adjacent antinodes belonging to a mode pattern of the room air volume. This means a higher probability of room air modes interfering with full development of the driven/radiating area modes. Therefore, the mode patterns of air volume reduce the effective output at certain resonance frequencies. Since the transducer fails to excite the air in the room at certain frequencies, and since sound is processed through coupling of transducer to air, this irregularity affects the tonal quality of the input reaching the listener's ears.
A fifth deficiency typical of stretched thin-film transducers relates to the unstable behaviour of a large-area oscillating thin film at low frequencies. The mass of the thin film is so low when compared to the mass of the air in the room that the effective mass must be treated as the sum of the two masses. This means good thin-film to air impedance matching for relatively small rooms. This also means that the effective output at low frequencies will be a function of the room volume. In a small room, the oscillating thin film interacts with the entire room air mass, whereas in larger rooms it decouples itself from part of the contained air mass due to increased compressibility of the larger volume of air. This results in poor thin-film-to-air-mass impedance matching for relatively large rooms.
A proposed solution to solving some of the problems of a large oscillating area is in U.S. Pat. No. 4,156,801 to R. C. Whelan et al. (May, 1979), the disclosure of which is incorporated herein by reference, disclosing a thin-film transducer with a centrally driven/radiating area that is small when compared to the nondriven area. The nondriven area is baffled in order to minimize the effect on the radiated output of the nondriven area moving out-of-phase with the centrally driven area. Additional intentions, as stated by the inventors, were to exploit dimensions of the driven/radiating area for better sound dispersion, radiation energy, bandwidth and other parameters. A side benefit of a driven/radiating area positioned in a central opening of a relatively large baffle is the reduction of back-to-front-wave-leakage phase cancellations at low frequencies due to increased back to front effective wave path. For baffle dimensions much larger than the driven/radiating area and much larger than the wavelength of emitted sound, the back and front of the transducer act independently. However, at low resonance frequencies above the fundamental, antiresonance minima already reduce effective output to such an extent that the baffle ceases to play any significant role in preventing back-to-front-wave-leakage phase cancellations. Despite the existence of a relatively small driven/radiating area, pronounced low frequency antiresonance minima will still occur because the intrinsic symmetry of the centrally positioned driven/radiating area, in combination with:
will still give rise to phase cancellations from out-of-phase displacements of adjacent antinodes that are exposed through the central opening (open area) in the baffle.
A further reduction in strength of effective output, and a consequent degradation of the tonal quality, originates in the central area being driven by forces that act in conflict with the out-of-phase displacements of adjacent antinodes. The spectrum of the radiated waveform will be poor in frequency components associated with such adjacent antinodes. And one last deficiency caused by the symmetrically (centrally) positioned driven/radiating area is an equal back-to-front-wave-path of all possible paths, giving rise to strong destructive interference for comparable wavelengths of emitted sound. Such destructive interference is manifested as an additional amplitude dip in the frequency response or as an additional gap in the spectrum of a complex waveform, with audible degradation in tonal quality of effective output. Hence, a symmetrically (centrally) positioned driven/radiating area in a relatively large oscillating thin film, combined with a symmetrically (centrally) positioned opening in a relatively large baffle, offers limited improvements to tonal quality of the transducer.
A sixth deficiency of some thin-film transducers originates in the need for a transformer or other circuitry in order to match a significant impedance difference with the amplifying device, rendering impossible the benefits and simplicity of direct coupling.
A seventh deficiency, typical of stretched thin-film transducers with magnetic field distributed over one side of the thin film, is distortion due to nonlinearity of the magnetic (or electrostatic) field. In particular, for large displacements (excursions), the conductors adhered on the thin film are exposed to a weaker magnetic field because of larger distances from the magnet assembly. Hence, for a given current intensity, the electromagnetic forces will be much weaker or much stronger at extreme positions compared to average displacements of the oscillating medium. A side effect of magnetic structures is the magnetic field intensity losses due to pole pieces (usually perforated ferromagnetic plates) supporting the magnet assembly. Losses in internal magnetomotive force result from the magnetic reluctance in the pole piece and in the inevitable gap where the pole piece joins the magnet. A pole piece is necessary for directing a maximum magnetic flux to the conductors adhered to the thin film.
An eighth deficiency typical of all thin-film transducers is the interference of magnetic or electrostatic motor structures with the process of reproducing a waveform and the process of radiating a waveform:
The interference of such structures with the process of producing a waveform relates to strong coupling of the oscillating thin film with air. The free motion of the oscillating thin film becomes a function of the displaced air volume flowing through the structure per unit time that, in turn, depends on the total cross-sectional area of air-flow spacing. The smaller the cross-sectional area of air-flow spacing, the smaller the volume of air flowing per unit time and the stronger the interference of such structure with the free motion of the oscillating thin film. For example, in a structure composed of a magnet assembly mounted on a perforated pole piece, the total air-flow spacing area is determined by the total area of perforations not covered by the assembly of magnets. In practice, the compromise between ferromagnetic and mechanical properties of the pole piece, minimisation of magnetic flux losses and total air-flow spacing cross-sectional area constitutes an obstacle in achieving an optimum air flow through the magnet structure. Thus, the magnet structure interferes with the free motion of the oscillating thin film that, in turn, affects the reproduced waveform, therefore the tonal quality of the transducer. An extreme case of a structure interfering with free motion of the oscillating thin film is the flapping of the thin film against such structure at high amplitude oscillations.
The interference of structures with the process of radiating a waveform relates to obstructing the propagation of waves in air. Transducers with such structures on each side of the oscillating medium, interfere with wave propagation of both front and rear waves. Transducers with structures on one side of the oscillating medium, interfere with the propagation of waves on that side only. Since radiated waves propagate as air disturbances induced by thin-film oscillations, such structures act as obstacles that interfere with free propagation of waves, thus degrading the tonal quality of reproduced sound.
A ninth deficiency characteristic of electromagnetic thin-film transducers relates to a predominantly parallel, in lengthwise or widthwise direction, sparse distribution of conductors and magnetic fields throughout the driven/radiating area. The resulting sparse distribution of the exerted electromagnetic forces gives rise to nondriven long parallel strips oscillating out-of-phase with driven long parallel strips. Minima in effective output will occur at all frequencies by out-of-phase cancellations from driven strips alternating with nondriven strips.
To sum up, main sources of audible effects on tonal quality are:
For rigid-diaphragm transducers: inefficiency in reproduced output of hermetically sealed enclosures, coloration caused by air resonances in open enclosures, pronounced variations in effective output caused by diaphragm modes of oscillation at resonance frequencies, voice coil impedance, crossover impedance, crossover resonances, and crossover losses.
For thin-film transducers: tonal quality and resonance-antiresonance irregularities in effective output at resonance frequencies of the oscillating thin film, pronounced back-to-front-wave-leakage phase cancellations and back-to-front-wave-path destructive interference for comparable wave length of radiated sound, dependence of thin-film low end frequencies on room volume, cancellations in effective output because of the thin film spanning adjacent out-of-phase room air antinodes, pronounced directionality, interference of driving structures with radiation of waves, out-of-phase motion of driven and nondriven sections of the oscillating medium, flapping against such structures at high amplitudes, low frequency oscillations of thin film, nonlinear magnetic field for magnet structures positioned on only one side of the thin film, crossover resonances, crossover impedance and crossover losses, weak mechanical coupling of conductors with oscillating medium, sparse distribution of exerted forces throughout the driven/radiating area.
With all their drawbacks, rigid diaphragm transducers with enclosures have been dominating the high-fidelity market because of their excellent performance to price ratio. Tonal quality, though affected by enclosure coloration, is rich in overtones, giving rise to warm, pleasant sound. On the other hand, thin-film transducers are low distortion devices with virtually no coloration but poor in tonal quality, unbalanced, because the low frequency content, though possibly present, is inaudible.
Consequently, a need exists for a transducer with reduced irregularities in effective output for the purpose of achieving a closer approximation to natural tonal quality of reproduced sound.